One of the fundamental objectives of optical data storage research has been the generation of small and intense optical spots. This objective has become even more pertinent to the magnetic data storage industry with the conceptualization of a heat assisted magnetic recording system. Some devices for generating small optical spots use: a focusing device, such as a lens, that bends the optical rays toward a common point; small apertures in metal that generate evanescent fields; or a combination of the two.
Heat assisted magnetic recording (HAMR) has been proposed as a means by which the recording density of hard disc drives may be extended to 1 Tb/in2 or higher. Current conventional hard disc drive technology is limited by the superparamagnetic effect, which causes the small magnetic grains needed for high density recording media to gradually lose their magnetization state over time due to thermal fluctuations. By using heat assisted magnetic recording, the magnetic anisotropy of the recording medium, i.e. its resistance to thermal demagnetization, can be greatly increased while still allowing the data to be recorded with standard recording fields. In HAMR, a laser beam heats the area on the disc that is to be recorded and temporarily reduces the anisotropy, and hence coercivity, in just that area sufficiently so that the applied recording field is able to set the magnetic state of that area. After cooling back to the ambient temperature, the anisotropy returns to its high value and stabilizes the magnetic state of the recorded mark.
HAMR systems require a device that is able to conduct sufficient light energy into the recording medium to heat it by several hundred degrees, but only in the area that is desired to be recorded, which typically will have dimensions on the order of a single bit, which is about 25 to 50 nm, if the recording density is 1 Tb/in2. If the optical hot spot is larger than this area, it will extend to neighboring bits and tracks on the disc, and by heating those areas as well, the data recorded in those areas will be erased.
Most focusing systems used in optical data storage systems can be thought of as generating a converging cone of optical rays from a narrow group of parallel rays. If θ is the half angle of the cone, the numerical aperture (NA) of the optical system is given by n sin θ, where n is the refractive index of the medium in which the vertex of the cone resides. For a given optical frequency, diffraction theory predicts that the size of the focused spot is inversely proportional to the numerical aperture of the system.
This invention provides an optical transducer design that accommodates non-planar incident wavefronts and avoids the need for phase shifting a portion of the incident light.